This invention relates to a system and a method which together are capable of analyzing or assessing to a very high degree of resolution the effects of over- and under-exposure of photographic film negative and print optical sound tracks (e.g. such as for motion picture film) so as to miniminize or eliminate the effects of exposure on image size. Although the system is designed to be used in conjunction with the emerging technology of digital optical sound tracks, it is equally applicable to traditional industry standard analogue optical sound tracks.
Motion pictures have historically employed a variety of types of sound tracks, involving both optical and magnetic recording media. Standard industry practice for the 35 mm theatrical release format has for many years been the so-called variable area optical sound track. In accordance with this standard practice, a 1/10" wide track, alongside the picture area, is dedicated to the optical sound track. In general, sound is recorded on the film by exposing the area dedicated for the sound track to a source of light (visible or otherwise) so that the sound track comprises a portion which is essentially opaque and a portion which is left essentially transparent, the ratio between the two portions being proportional to the instantaneous amplitude of the sound signal being recorded. In reproduction, a light source is focused through a slit onto this track, on the far side of which is a photo-sensitive solar cell. The electrical output of the cell is an instantaneous measure of the amount of light passing through the film, and is thus a function of the amplitude of the sound being reproduced.
As is true with the picture element of a motion picture film, the sound element is first recorded on a negative, and is then printed onto a positive, which is subsequently distributed to theatres. In order to achieve an adequate measure of opacity in the print so as to achieve acceptable electrical signal levels in the sound reproducer, the print must be over-exposed. In order to maintain an adequate level of transparency under these over-exposure conditions, the negative must also be over-exposed. A consequence of this is image spread, i.e. the tendency for an image to grow in size in relation to the length of time of exposure. The exposed areas tend to grow, to the detriment of the unexposed areas. This characteristic, if uncompensated, would result in distortion of the sound. Fortunately, since exposed areas on the positive correspond to unexposed areas on the negative, the two effects can be made to cancel. Corrections can also be made for light scatter, film grain, and any other effects which tend to result in inaccuracies in the image.
An ingenious method for determining the optimum exposure conditions for both negative and print so as to result in cancellation of all deleterious effects on image size was described by Baker and Robinson, and has become industry-standard practice (J. V. Baker and D. H. Robinson, "Modulated High-Frequency Recording as a Means of Determining Conditions for Optimal Processing," Journal of the Society of Motion Picture Engineers, Vol. 30, p. 3, Jan. 1938). This method has come to be known as the "cross-modulation test". It is used routinely as a quality assurance mechanism prior to actual sound recording to verify the capability of the process to eliminate the effects of component (particularly exposure lamp) aging, changes in film characteristics from production batch to production batch, and so forth.
The above mentioned "cross-modulation test" is based on the fact that a perfect sinusoid comprising a high frequency signal modulated by a low-frequency one, will have an average value of zero. In the case of a perfect optical image of such a sinusoid, the average light transmission will be constant. In the case of underexposure or overexposure, some of the low-frequency modulation component will be introduced into the average value of the signal and may be detected. In practice a high frequency carrier at about 10 Kilohertz is modulated at about 75% by a 400 Hertz signal. The resulting image is played back through a low-pass filter to eliminate the high frequency carrier, and the amount of 400 Hertz signal remaining is analyzed to determine the exposure and printing conditions which result in the lowest-level signal.
In recent years, a number of proposals have been put forth to replace the traditional optical analogue sound track with an optical digital sound track. Potential methods for recording and reading a digital soundtrack on motion picture film are, for example, described in U.S. Pat. No. 4,600,280; such methods have the basic capability of recording and reading information as a series of linear arrays composed of opaque and transparent spots, representing binary ones and zeroes.
An optical digital soundtrack would replace the continuously-varying waveform presently recorded as an analogue of sound pressure level, with a pattern of minuscule opaque and transparent spots representing a binary number whose value is a measure of the instantaneous sound pressure level. The use of digital recording techniques is expected to duplicate the considerable improvements in quality and longevity that have been achieved in the migration from (analogue) vinyl long-play records to (digital) Compact Discs in the home entertainment industry.
The recording and reproduction process for digital sound tracks, however, will require even greater accuracy in image reproduction than is required for analogue sound tracks.